In a broad sense, biomass comprises all plants and microbes, as well as animals employing the plants and microbes as food and wastes produced by the animals; and in a narrow sense, the biomass mainly means substances such as lignocellulose (lignin for short) such as stalks and trees except for grains and fruits in a farming and forestry production process, leftovers of the agricultural product processing industry, farming and forestry wastes, livestock and poultry feces and wastes of an animal husbandry production process. Common representative biomass comprises crops, crop wastes, timber, timber wastes, animal feces, etc. The biomass becomes an important component of renewable energy sources due to the characteristics of renewability, low pollution and widespread distributivity, so that how to efficiently develop and utilize biomass energy plays very positive roles in solving problems on the energy sources and ecological environment.
At present, modes for developing and utilizing the biomass energy are mainly manifested in the following several aspects: (1) direct combustion: the biomass is directly burnt to use heat or further generate electricity; (2) a physical-chemical process: the biomass is subjected to physical-chemical treatment, followed by compression-forming, to obtain a formed fuel; (3) a biochemical process: the biomass is hydrolyzed and fermented to obtain ethanol or is subjected to a biogas technology to obtain a biogas; and (4) a thermochemical process: the biomass is gasified to obtain a biomass fuel gas, or is pyrolyzed to obtain charcoal or a crude biomass oil, or is directly liquefied to obtain a liquefied oil, etc. wherein, during the direct liquefaction of the biomass to obtain the liquefied oil, the biomass directly reacts with hydrogen generally under the conditions of a high pressure and a catalyst and is converted into a liquid fuel.
For example, a method for direct hydroliquefaction of biomass, which comprises two boiling-bed hydro-conversion steps, is disclosed by a Chinese Patent Document CN 102127462 A. The method comprises the following steps: a) preparing a suspension of biomass particles in a solvent, preferably a hydrogen donating solvent; b) subjecting the suspension to first hydro-conversion in the presence of hydrogen in at least one reactor which contains a boiling-bed catalyst and performs operation under the conditions of a temperature of 300 DEG C. to 440 DEG C., a total pressure of 15 MPa to 25 MPa, a hourly mass velocity of 0.1/h to 5/h and a hydrogen/feed ratio of 0.1 Nm3/kg to 2 Nm3/kg; and c) subjecting at least part of an effluent obtained in the step b) to second hydro-conversion in the presence of hydrogen in at least one reactor which contains a boiling-bed-form catalyst and performs operation under the conditions of a temperature of 350 DEG C. to 470 DEG C., a total pressure of 15 MPa to 25 MPa, a hourly mass velocity of 0.1/h to 5/h and a hydrogen/feed ratio of 0.1 Nm3/kg to 2 Nm3/kg. In the above-mentioned technology, a biomass feedstock is directly liquefied into a liquid substance through the first hydro-conversion step and the second hydro-conversion step.
In the above-mentioned technology, the liquid substance is separated to obtain a gas phase, an aqueous phase, as well as a light fraction comprising at least one selected from a group consisting of naphtha, kerosene and/or diesel liquid hydrocarbons, a vacuum gas oil heavy hydrocarbon fraction, a vacuum residue fraction and a solid fraction capable of being in a vacuum residue. The light fraction can be mingled with the vacuum gas oil heavy hydrocarbon fraction and the vacuum residue fraction during separation, and thus, the yield of the light fraction is lowered; and heavy fractions can be mingled with the light fraction during separation, and thus, the yield of the light fraction is also affected.